1. Field of the Invention
The present invention relates generally to a grinding and dispersing method and apparatus and, more particularly, to an improvement in basket mills in which a material to be treated having solid constituents is finely ground and dispersed in a liquid using grinding media.
2. Background Information
Basket mills are apparatuses by which a material to be treated (mill base) is ground and dispersed in a concentrated manner using a small amount of grinding media. Grinding and dispersing apparatuses of this type are described in, for example, JP-B-59-46665 (JP-A-58-174230), JP-B-62-16687 (JP-A-60-48126) and JP-B-5-82253 (JP-A-1-210020). However, in the grinding and dispersing apparatuses disclosed in these publications, a container containing dispersing media is used as a basket which is disposed in a tank containing the material to be treated, and a stirring shaft connected to a bearing passes through a hole at a bottom portion of the basket and extends downwardly from the basket. Accordingly, in the foregoing conventional apparatuses, there is the possibility that the material to be treated clogs a trunk portion of the basket, the grinding media clogs the hole at the bottom of the basket through which the stirring shaft extends, the basket is subjected to degradation due to heat generation during grinding and dispersion of the material to be treated, and the basket and stirring shaft bearing are subjected to abrasion causing the grinding media to flow into the tank.
Conventional grinding and dispersing apparatuses wherein a stirring shaft does not extend downwardly through a bottom portion of a container disposed within a tank are disclosed in, for example, JP-A-2-207853, JP-B-6-73620 (JP-A-6-86924). In such apparatuses, a rotor is disposed at the bottom portion of the container, and a gap is formed between a lower end face of the container and the rotor to allow only the material to be treated to flow out of the gap. Furthermore, in a conventional grinding and dispersing apparatus disclosed in JP-B-8-17930 (JP-A-3-72932), a bottom of a container is formed as a blind bottom panel, and only a material to be treated is allowed to flow out of small pores disposed at side walls of the container. In these conventional apparatuses, however, there is a tendency toward clogging and/or heat generation, and the circulation of the material to be treated within the container sometimes becomes inadequate.
Other conventional apparatuses of the type set forth above provide a jacket disposed in a container within a tank to eliminate the generation of heat, as described in JP-A-56-37057 and Utility Model JP-A-4-87742 (Utility Model No. 2515678). In these conventional apparatuses, a partition such as a screen is disposed at each of both sides (upper and lower sides) of the container to form a space containing grinding media within the container, and blades of an axial flow pump are attached to a stirring shaft which extends towards the outside (lower side) of the screen to allow flow of the material to be treated. Accordingly, these conventional apparatuses still suffer from problems of clogging and outflow of the grinding medium at portions of the container where the stirring shaft passes through the screen, and the material to be treated does not adequately flow in or flow out of the space containing the grinding media.
Moreover, other conventional grinding and dispersing apparatuses are constructed so that a container is positioned within a tank, and stirring blades for stirring a fluid are disposed at a lower portion of the container, whereby a material to be treated will flow from the lower portion toward an upper portion of the container, and the material to be treated enters the container at a central portion of the tank. In such apparatuses, the material to be treated is not fed directly into the container, thereby making the feeding process of the material to be treated uncertain.